More work to make the Expense page arbitrary for re-use.

lib/models/tracked_type.dart

@@ -11,6 +11,9 @@ abstract class TrackedType {
     required this.description,
   });
 
+  static String amountText = "Amount";
+  String getAmountText() => amountText;
+
   @override
   String toString() {
     return toMap().toString();